The sp 2 has more s character than the sp 3 , the electrons in the s orbital are closer to the nucleus and are strongly attracted to it, so that an sp 2 has a tendency to attract electrons towards itself, which generates dipole moments.

In trans alkenes the dipole moments are subtracted, even canceling out in the event that both carbons have the same chains. In cis alkenes the dipole moments add up giving rise to a non-zero total dipole moment (polar molecule)